1. Field of the Invention
The invention relates to a vehicular constant-speed control apparatus and a method of controlling vehicle speed and, more particularly, to a vehicular constant-speed control apparatus that controls operation of an internal combustion engine and gear-shift stages of a transmission such that a vehicle runs at a constant speed, i.e., at a target vehicle speed, and to a method of controlling vehicle speed.
2. Description of Related Art
Conventionally, as a vehicular constant-speed control apparatus of this type, there has been proposed one which downshifts a transmission if a vehicle speed deviation representative of a difference between a vehicle speed and a target vehicle speed has become equal to or greater than a predetermined value. One such constant-speed control apparatus is disclosed e.g. in Japanese Patent Application Laid-Open No. 7-172209. In this constant-speed control apparatus, a transmission is downshifted while a vehicle runs at a constant speed. Thus, an increase in vehicle speed is prevented even if an engine is in operation with a full-closed throttle valve while the vehicle runs down a downhill.
However, according to such a vehicular constant-speed control apparatus, even in the case where a downshift causes an immediate decrease in vehicle speed while a vehicle runs down a relatively gentle slope, the downshift is performed if a vehicle speed deviation between a vehicle speed and a target vehicle speed has become equal to or greater than a predetermined value. In this case, the speed of the engine needs to be increased to maintain the vehicle speed after the downshift at the target vehicle speed. This causes problems of an increase in the amount of fuel consumed by the engine and deterioration of engine noise levels.
The invention has been made as a solution to the above problems. It is an object of a vehicular constant-speed control apparatus and a method of controlling vehicle speed according to the invention to restrain an increase in fuel consumption and deterioration of engine noise level while a vehicle runs at a constant-speed.
A vehicular constant-speed control apparatus according to the invention controls operation of an internal combustion engine and gear-shift stages of a transmission such that a vehicle equipped with the internal combustion engine and the transmission runs at a constant speed, i.e., at a target vehicle speed, and comprises a vehicle speed sensor that detects a vehicle speed, an acceleration sensor that detects an acceleration of the vehicle, and an operation state detector that detects an operation state of the internal combustion engine. The transmission is downshifted if the operation state detector detects that the internal combustion engine is in an operation state of a full-closed throttle valve and if the acceleration detected by the acceleration sensor is equal to or greater than a criterion value. The criterion value is set as a value that decreases as a vehicle speed deviation between the vehicle speed detected by the vehicle speed sensor and the target vehicle speed increases.
In the vehicular constant-speed control apparatus according to the invention, the transmission is controlled to be downshifted if the operation state detector detects that the internal combustion engine is in an operation state of a fully-closed throttle valve and if the acceleration detected by the acceleration sensor is equal to or greater than a criterion value that decreases as the vehicle speed deviation between the vehicle speed detected by the vehicle speed sensor and the target vehicle speed increases. Thus, in the case where the vehicle speed has increased on a steep downhill, the acceleration of the vehicle is great even if the vehicle speed deviation is small. Therefore, a downshift can be performed quickly. In the case where downshift causes an immediate decrease in vehicle speed while the vehicle runs down a relatively gentle slope, a downshift is not performed until the vehicle speed increases to a measurable value. This eliminates the possibility of an unnecessary downshift being performed to prevent the vehicle speed from increasing. That is, it is possible to downshift the transmission more appropriately, prevent an increase in the vehicle speed deviation between the vehicle speed and the target vehicle speed, and restrain an increase in fuel consumption and deterioration of the engine noise level.